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Prof. Sébastien R. A. Foucaud
Department of Earth Sciences
National Taiwan Normal University
Introduction to Astronomy
Unless noted, the course materials are licensed under Creative Commons
Attribution-NonCommercial-ShareAlike 3.0 Taiwan (CC BY-NC-SA 3.0)
SyllabusAstronomical techniques
• Lecture 1: Finding its way on the night…
• Lecture 2: Moving blindly and seeing the light…
• Lecture 3: Eyes better than my eyes: the telescopes
The solar system
• Lecture 4: Home: Earth
• Lecture 5: Our close friend: The Moon
• Lecture 6: Our neighborhood: Rocky Planets
• Lecture 7: The good Monsters: Giant Planets
• Lecture 8: Dwarf Planets, minor bodies and scenario of solar system formation
Stars and planets
• Lecture 9: Our king: The Sun
• Lecture 10: Shades of colors: so many stars…
• Lecture 11: The life of a star: from its birth to its death
• Lecture 12: The quest for another Earth: extrasolar planets
The Milky-Way, galaxies and our Universe
• Lecture 13: The cosmic carrousel: galactic structure & Galaxy formation
• Lecture 14: Hubble, the expansion of the Universe and the Big-Bang Theory
• Lecture 15: Einstein and the relativity2
3
Finding its way on the night…
Lecture 1
4
Outline
• Where are we?
• Measuring time
• Finding our way in the sky I:
the constellations
• Celestial sphere
• The ecliptic
• The seasons
• Finding our way in the sky II:
coordinate systems
• Light travels at a finite speed, thus it takes
time to travel distances
• Light from Sirius takes 8 years to reach us
• Light from the Orion Nebula takes 1500
years to reach us
• Light from Andromeda takes 3 million years
to reach us
5
Light and Seeing the Past
6
Where are We?
You are
here!
• Even nearby planets are distant compared to normal “human” scales
• Stars are very far away– Would take Voyager 1 100,000 years to reach Alpha
Centauri
• Galaxy even larger– 100,000,000,000 stars
• Universe contains 100,000,000,000,000 galaxies
7
Scale of the Universe
The Earth Rotates
0 km/hr
830 km/hr
1275 km/hr
1550 km/hr
1650 km/hr
Wikipedia Wikiscient
Star Trails Above Mauna Kea
Credit & Copyright: Peter Michaud (Gemini Observatory), AURA, NSF
APOD: 2005 December 20 9
• Due to the rotation of the Earth around its axis
• Sky appears to rotate east to west about the celestial poles
because Earth rotates west to east
• Period of rotation:
1 sidereal day= 23h56m4.1s
• 1 solar day (Noon to Noon) =24h
• Stars rotate around the North Star – Polaris
10
Daily Rising and Setting
Wikipedia Andreas Willems
11
Solar vs Siderial Day
Encyclopæ dia Britannica, Inc.
Solar vs Siderial Day
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Wikipedia Gdr
Earth’s orbit causes a difference between solar and sidereal time
13
Solar vs Siderial Day
Wikipedia Francisco Javier Blanco González País: ESPAÑ A
360o/365days = ~1 deg/day
1 deg = 24h x60min /360o = 4 min/day
stars rise 4 min earlier each day
4 x 30 days = 2hrs/month
• Sidereal Day: 23 hr 56 min 4 secMotion relative to background stars
• Mean Solar Day: 24 hoursThe average time between meridian crossings of the Sun
• Apparent Solar Day: variesThe actual time between the meridian crossings
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An Earth Day
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Moon Orbits the Earth
Average Earth-Moon distance = 380000 km
Wikipedia Thomas Splettstoesser
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Lunar Phases
• Sidereal rotation period is the same as the
sidereal orbital period: 27.3 days
• Moon shows the same “face” to the Earth
• Caused by Earth’s tidal force on the Moon
17
Moon’s Rotation Matches Orbit
Wikipedia Orion 8
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Wikipedia Luc Viatour
• Based on the motion of the
Moon around the Earth
• Sidereal month:
27.321662 days.
Defined relative to the
stars.
• Synodic month:
29.530589 days
Phase period; New Moon
to New Moon
19
Lunar Month
Scott R. Anderson, Ph.D.
This work is licensed by “University of Chicago Scott R. Anderson,
Ph.D. Introduction to Astronomy course” for the use of “Course
Database of General Education TW” ONLY. The copyright belongs to
the above mentioned entity and GET does not have the right to sub-
license. Copyright privileges have to be negotiated with the copyright
owner(s) for separately.
20
Earth Orbits SunAverage orbital speed
=108,000 (km/hr)
Wikipedia following Duoduoduo's advice, vector image: Gothika.
• Sidereal Year: 365.256363 mean solar days.
Defined by the motion relative to stars.
• Anomalistic Year: 365.259635 mean solar days.
Defined as perihelion to perihelion.
• Tropical year: 365.242190 mean solar days.
Defined from equinox to equinox. Thus, related to
the seasons; 20 minutes shorter than sidereal
year.
• Thus, your year off by a day every 72 years
21
An Earth Year
• Based on the motion of the Sun on the sky or the
motion of Earth around the Sun
• Egyptian calendar had 365 days, resulting in a
shift of equinoxes by 1 day every 4 years
• Julius Caesar introduced leap years in 50 BC
• Julian Year: 365.25 mean solar days.
Used in calendars before 1582.
• Equinoxes still shifting over periods of centuries
• Pope Gregory XIII modified the leap years to
account for this
• Gregorian Year: 365.2425 mean solar days.
Used in calendars today. 22
An Earth Year
• Depends on where you are on the Earth!
• Time zones ensure that the noon is really noon, i.e. sun is at highest point
• To avoid confusion, use universal time (UT), the time at the meridian in Greenwich
UT = CST - 8 hrs
23
What time is it?
Established to insure that sun is at highest point approximately at noon in the middle of the time zone
24
The Time Zones
Wikipedia Phoenix B 1of3
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Celestial points
Finding the north celestial pole Finding the south celestial pole
Wikipedia Uploader: Micke
• ~5000 stars visible with naked eye
• ~3500 of them from the northern hemisphere
• Stars grouped together into constellations since antiquity
• Officially 88 constellations determined by the IAU
• Names range from mythological (Perseus, Cassiopeia) to technical (Air Pump, Compass)
28
Stars and Constellations
Wikipedia IAU and Sky & Telescope magazine
(Roger Sinnott & Rick Fienberg)
29
Orion Constellation
Wikipedia Mouser
30
Orion Constellation
National Taiwan Normal University Kuan Chen
31
Orion Constellation
Wikipedia Uploader:
Urania’s Muse
National Taiwan Normal University Kuan Chen
Wikipedia Mouser
Wikipedia Created by Yours Truly, original JPEG uploaded by Montrealais, vectors by Mysid.
32
Orion Constellation
BetelgueuseBellatrix
Rigel
Saiph
Orion belt
33
Utility of Constellations
Wikipedia Torsten Bronger
34
Asterism
Wikipedia Uploader: DannyZ
35
National Taiwan Normal University Kuan Chen
The Celestial Sphere
National Taiwan Normal University Kuan Chen
36
Rotating Celestial Sphere
• An imaginary sphere surrounding the earth, on which we picture the stars attached
• Axis through earth’s north and south pole goes through celestial north and south pole
• Earth’s equator
Celestial equator
37
The Celestial Sphere
National Taiwan Normal University Kuan Chen
38
Circumpolar Constellations
National Taiwan Normal University Kuan Chen
The Local Sky
• This is the sky that you personally observe
– Horizon
– Zenith
– Meridian
39National Taiwan Normal University Kuan Chen
40
The Celestial Sphere
Nick Strobel
This page was copied from Nick Strobel's Astronomy Notes. Go to his site at
www.astronomynotes.com for the updated and corrected version.
41…depends where you are!
Your Local Sky…
Wikipedia Fcn
Horizon system
42
Your Local Sky…• The Horizon (altaz) System of Coodinates
• horizon is the reference frame
• altitude is height above horizon (0-90o)
• azimuth = angle measured East along horizon from north point (0-360o)
National Taiwan Normal University Kuan Chen
43
Relevant Geography
Wikipedia Marvel
44
The Sky at the North Pole
• At the North Pole, the North Celestial
Pole is at the zenith
• Stars never rise or set
• Planets, Moon, and Sun do rise and set
National Taiwan Normal University Kuan Chen
45
The Sky at Our Latitude
National Taiwan Normal University Kuan Chen
46
Stars Rise and Set at the Equator
National Taiwan Normal University Kuan Chen
47
The Sky at Southern Latitudes
National Taiwan Normal University Kuan Chen
48
The Ecliptic
Wikipedia Tau’olunga
Libra
(Oct. 21)
Scorpio
(Nov. 21)
Sagittarius
(Dec. 21)
Capricorn
(Jan. 21)
Virgo
(Sep. 21)
Leo
(Aug. 21)
Cancer
(Jul. 21)
Gemini
(Jun. 21)
Taurus
(May 21)
Aries
(Apr. 21)
Pisces
(Mar. 21) Aquarius
(Feb. 21)
49
The Celestial Sphere
National Taiwan Normal University Kuan Chen
• The Earth’s rotation axis is tilted 23½ degrees
with respect to the plane of its orbit around the
sun (the ecliptic)
• It is fixed in space, so sometimes we look
“down” onto the ecliptic, sometimes “up” to it
Path around sun
Rotation axis
50
Another Complication: Axis Tilt!
Natioanl Taiwan Normal University Sébastien R. A. Foucaud
51
The seasons
Wikipedia Tau’olunga
52
Summer solstice
National Taiwan Normal University Kuan Chen
53Wikipedia Axialtiltobliquity/Dna-webmaster/Patricka
Solstices and equinoxes
54
Summer solstice
Wikipedia Tau’olunga
55
Winter solstice
Wikipedia Tau’olunga
Credit & Copyright: Anthony Ayiomamitis
APOD: 2005 December 21 56
Sunrise by Season
57
Sun Changes Declination
National Taiwan Normal University Kuan Chen
58
Back to the North Pole
National Taiwan Normal University Kuan Chen
59
Cold Arctic Regions
National Taiwan Normal University Kuan Chen
60
Land of the Midnight Sun
Wikipedia Yan Zhang/GUN
61
Back to the Equator
National Taiwan Normal University Kuan Chen
• In the Tropics, Sun is always
close to zenith
• In the Tropics, the sun
reaches the Zenith at some
point in the year
• Lots of heating due to
sunlight
62
Tropical Heat
National Taiwan Normal University Kuan Chen
National Taiwan Normal University Kuan Chen
63
Precession of the Equinoxes
“The dawning of the age of
Aquarius”
• The Earth is not a perfect sphere
• Things that are not perfect spheres wobble as they spin
• Earth wobbles on 26,000 year time scales
Wikipedia Dbachmann, Tom Ruen
PolarisVega
(14,000 yr)
64
North Celestial Pole Changes
Wikipedia Tau’olunga
Thanks!
65
Work Licensing Author/Source
NASA
http://map.gsfc.nasa.gov/universe/bb_cosmo_struct.html
2012/03/06 visited
NASA
http://www.spitzer.caltech.edu/images/1925-ssc2008-10b-A-Roadmap-to-the-Milky-
Way-Annotated-
2012/03/06 visited
Wikipedia Wikiscient
http://en.wikipedia.org/wiki/File:Globespin.gif
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NASA, Peter Michaud (Gemini Observatory), AURA, NSF
http://apod.nasa.gov/apod/ap051220.html
2012/03/06 visited
Wikipedia Andreas Willems
http://en.wikipedia.org/wiki/File:Rotation_of_earth.jpg
2012/03/06 visited
This work is from “Encyclopæ dia Britannica, Inc.”
http://kids.britannica.com/comptons/art-54083/When-the-Earth-is-at-position-1-in-
its-orbit It is used subject to the fair use doctrine of:
•Taiwan Copyright Act Articles 52 & 65
•Britannica Terms of Use
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Wikipedia Chris828
http://de.wikipedia.org/w/index.php?title=Datei:Sidereal_day_(prograde).svg&fileti
mestamp=20090211133832
2012/03/06 visited
Wikipedia Francisco Javier Blanco González País: ESPAÑ A
http://commons.wikimedia.org/wiki/File:Tiempo_sid%C3%A9reo_2.png
2012/03/06 visited
Wikipedia Mungany/Ankxo
http://commons.wikimedia.org/wiki/File:Orbit_of_Moon_es.jpg
2012/03/06 visited
Wikipedia Thomas Splettstoesser
http://en.wikipedia.org/wiki/File:Lunar_phase_diagram.png
2012/03/06 visited
Wikipedia Orion 8
http://en.wikipedia.org/wiki/File:Moon_phases_en.jpg
2012/03/06 visited
Wikipedia Luc Viatour
http://zh.wikipedia.org/wiki/File:Full_Moon_Luc_Viatour.jpg
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Copyright DeclarationWork Licensing Author/Source
This work is licensed by “University of Chicago Scott R. Anderson, Ph.D.
Introduction to Astronomy course” for the use of “Course Database of General
Education TW” ONLY. The copyright belongs to the above mentioned entity and
GET does not have the right to sub-license. Copyright privileges have to be
negotiated with the copyright owner(s) for separately.
•The material on this website may be reused as described under the Open Course
License 0.1
Wikipedia following Duoduoduo's advice, vector image: Gothika.
http://en.wikipedia.org/wiki/File:Seasons1.svg
2012/03/06 visited
Wikipedia Phoenix B 1of3
http://en.wikipedia.org/wiki/File:Worldwide_Time_Zones_(including_DST).png
2012/03/06 visited
Wikipedia Bartolomeo and Christopher Colombus
http://en.wikipedia.org/wiki/File:ColombusMap.jpg
2012/03/06 visited
Wikipedia Abraham Ortelius
http://en.wikipedia.org/wiki/File:PeriplusAncientMap.jpg
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Wikipedia NASA/HST
http://en.wikipedia.org/wiki/File:Polaris_alpha_ursae_minoris.jpg
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http://en.wikipedia.org/wiki/File:Pole01-eng.jpg
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Wikipedia IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
http://en.wikipedia.org/wiki/File:Orion_IAU.svg
2012/03/06 visited
Wikipedia Mouser
http://en.wikipedia.org/wiki/File:Orion_3008_huge.jpg
2012/03/06 visited
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http://en.wikipedia.org/wiki/File:Orionurania.jpg
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Wikipedia Created by Yours Truly, original JPEG uploaded by Montrealais, vectors
by Mysid.
http://en.wikipedia.org/wiki/File:Orion-guide_dark.svg
2012/03/06 visited
Wikipedia Torsten Bronger
http://zh.wikipedia.org/wiki/File:Andromeda_constellation_map.svg
2012/03/06 visited
Wikipedia user: Skatebiker
http://en.wikipedia.org/wiki/File:Coathanger.png
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http://en.wikipedia.org/wiki/File:Collinder_399.jpg
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National Taiwan Normal University Kuan Chen
This work is from “Nick Strobel” http://www.astronomynotes.com/nakedeye/s5.htm.
It is used subject to the fair use doctrine of:
•Taiwan Copyright Act Articles 52 & 65
•This page was copied from Nick Strobel's Astronomy Notes. Go to his site at
www.astronomynotes.com for the updated and corrected version.
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